Older actions in other workflows that do not use credentials manager are still working.

Currently I am trying to terminate a workflow, but this action usually returns ServiceNoResponse instantly (took a few minutes to fail on one attempt).

I configured this connection using Chrome. Initially I tried IE but ran into another known bug where it offered a file to download rather than create the connection.

Workflow has been published/run in Chrome and IE, no difference.

Probably an unrelated bug, but I also had issues with the connections clearing themselves from actions. Have not been able to reproduce this at will, but sometimes when I go to publish a workflow an error pops up because all start/terminate workflow actions do not have connections and I have to mass reapply these with the "Apply connection to all" checkbox again.

The Connection Manager errors (ServiceNoResponse, Unauthorized request for connection) were red herrings. The issue was in the Terminate Workflow Instance action which had been passed the Item URL rather than the Site URL.